This title is aimed at providing a coherent, essentially self-contained, rigorous and comprehensive abstract theory of Feynman's operational calculus for functions of (typically) noncommuting operators. Although it is inspired by Feynman's original heuristic suggestions and time-ordering (or disentangling) rules in his seminal 1951 paper, as is made clear in the text, the theory developed in this book also goes well beyond them in a number of directions which were not anticipated in Feynman's work.